Diffractive optical elements (“DOE”) are used in many applications, such as optical storage devices, processing, sensing, and communications. A DOE serves to wave-shape incoming light. Whereas standard refractive optical elements, such as mirrors and lenses, are often bulky, expensive and limited to a specific use, DOEs are generally light-weight, compact, easily replicated, and can modulate complicated wavefronts.
A Fresnel lens is an example for a DOE. Like a classical convex lens, the Fresnel lens focuses parallel light onto a single focal point. The Fresnel lens' design can be considered conceptually as being created by removing slabs of glass that do not contribute to the bending of light rays to the focal point. Conventional Fresnel lens fabrication requires that the lens profile needs to be etched into, for example, a glass wafer. This fabrication is done by step-wise. The first etched profile, a so-called Fresnel plate, is not a good approximation of the ideal Fresnel shape. A conventional Fresnel plate is only ˜41% effective (i.e., only around 41% of transmitted light gets focused).
Other examples of DOEs include beam shapers, e.g., beam homogenizers. A DOE can also encode complex structures, which produce visible images in the far field when the light passes through the DOE structure. The fabrication of a DOE, which encodes the images, is a rather complex process. The gray-scale level of each image pixel is encoded by the phase shift of the light which passes through the DOE. This is achieved by encoding the depth of the features, which are etched in clear dielectric.
A typical DOE's consists of many features with a typical size of 7 μm, with depths varying continuously between 0 and 600 nm, to produce a phase shift between 0 and π for a 632 nm irradiation wavelength. The depth of the etched features must be precisely fabricated, and the etch roughness must be as low as possible. For example, the roughness of the etched features for a DOE, which encodes an image with 16 colors, must be smoother than 30 nm.
The three-dimensional (“3D”) surface profile of the DOE determines how the element will shape an incoming wavefront. Hence, the key feature of any DOE is its complicated 3D surface topography. Some gratings can be blazed or cut, but most DOE are made by micro-fabrication techniques. This usually involves a lithography step and an etching step: A photo-sensitive resist layer is applied and exposed with a mask under ultra violet (“UV”) light. After developing, the mask pattern is transferred into the resist layer. The resist layer then defines where material is etched away. This is normally done with reactive ion etching (“RIE”). The step depth is defined by resist layer profile and the RIE etch recipe.
Multiple levels are made by multiple photo-lithography and etching steps in standard, multilevel fabrication methods. However, such standard, multilevel fabrication methods work better in theory than in practice. For example, the use of multi photo-masks is a challenge because each mask must be aligned with respect to the previous etch step. The alignment is never perfect, generating displacements in x- and y-directions and rotational errors. With the number of lithography and etching step cycles the alignment errors add up. This is the reason why most DOEs only have 8-16 step levels. The theoretical efficiency (i.e., the fraction of the light that gets focused s) for a DOE with 8 steps and without fabrication errors like misalignment or surface roughness is no better than 95%. Fabrication errors will reduce this efficiency and further degrade optical performance. Increasing the number of steps in theory will increase the efficiency, but in practice, fabrication errors have been found to reduce the efficiency with larger effect.
An alternative, conventional micro-fabrication method to fabricate 3D profiles includes the use of standard gray-tone lithography. Gray-tone lithography (also known as gray-scale lithography) is a standard lithography process that results in continuously variable resist profiles. A gray-tone optical mask is used to transmit only a portion of the intensity of incident light, partially exposing sections of a positive photoresist to a certain depth. This exposure renders the top portion of the photoresist layer more soluble in a developer solution, while the bottom portion of the photoresist layer remains unchanged. The number of resolvable levels in gray-tone lithography has been limited by photoresist exposure nonlinearity, variability in development and material homogeneity to commonly around 16 levels under common conditions. With careful attention to detail, it is possible to achieve gray-tone lithography resolution of up to ˜80 levels. Continuous structures can be produced by heating the photoresist to smooth out surface non-uniformity.
The developed photoresist may be processed, for example by etching, to reproduce a scaled version of the three dimensional structure on the substrate. As the etch proceeds, the photo-resist mask slowly erodes, exposing the underlying dielectric to the high etch rate plasma. Thus, gray-scale technology relies on specifically developed RIE recipes to control the relative etch rate of the substrate called “etch selectivity”. This aspect defines the final vertical dimensions of a 3D structure.
There are multiple, standard methods for generating a gray-tone resist structure: diffuser-based, direct-writing (e.g. with a laser or electron beam), exposure through a High Energy Beam Sensitive (“HEBS”) glass mask, and traditional stepper exposure. Direct writing and stepper exposures can have so called “stitching errors”, because only a small field-of-view is exposed and multiple fields are stitched together. All gray-tone lithography methods have problems generating sharps drops or steps. Inevitably produced “transition regions” at the tops and bottom of steps scatter light and, therefore, degrade the DOE performance. Furthermore, for all gray-tone methods, the number of levels is limited to maximal ˜80.
All of the above-described techniques (e.g., multiple lithography/etching step cycles and gray-tone methods) have one additional and fundamental limitation: RIE etching is known to create surface non-uniformities or roughness, due to the random nature of the etching process and material inhomogeneity. This surface roughness will lead to scattering and degrade the DOE performance. The effect of the RIE-introduced roughness on DOEs depends on the wavelength of light. The shorter the wavelength is, the smaller the surface roughness must be.